1. Field of the Invention
The present invention relates to a motor control apparatus equipped with a function that corrects a torque command by calculating the amount of compensation for canceling out cogging torque which can cause an external disturbance in motor control (such a compensation will hereinafter be referred to as cogging torque compensation or cogging compensation).
2. Description of the Related Art
In recent years, the use of a direct-drive motor as a motor for driving the rotating axis of a machine tool has been increasing in order to meet the demand for higher speed and higher precision. Generally, in the case of an axis connected to a direct-drive motor, cogging compensation is applied in order to improve the smoothness of feed motion, since this type of motor produces a greater cogging torque than a conventional motor with a shaft. In the cogging compensation, the amount of compensation is calculated by performing frequency analysis on the torque command being output from a speed loop controller during constant slow-speed feed operation.
In the case of a feed axis in which the influence of gravity does not vary, the torque command being output during slow-speed feed operation is nearly constant and contains fluctuating components due only to the cogging torque; therefore, the amount of cogging compensation can be calculated by performing frequency analysis on the torque command.
On the other hand, in the case of an axis that causes a table to tilt, since the influence of gravity changes according to the tilt angle, the torque command being output during slow-speed feed operation is not constant but changes greatly according to the tilt angle. Since the torque command thus contains fluctuating components due to gravity, if frequency analysis is performed directly on the torque command, it is not possible to calculate the proper amount of cogging compensation.
In this way, there are cases where components due to other factors than cogging torque (for example, components due to gravitational torque, etc.) are superimposed on the torque command being output during constant slow-speed feed operation, and in such cases, there arises a problem that a proper amount of cogging compensation cannot be calculated if the frequency analysis is performed directly on the torque command. That is, if a component, such as a monotonically increasing or decreasing component are superimposed on the torque command, the proper amount of compensation cannot be obtained if the torque command is simply Fourier-transformed.
Patent document 1: Japanese Unexamined Patent Publication No. H07-284286
Patent document 2: Japanese Unexamined Patent Publication No. 2005-224051